All roads to schizophrenia lead to dopamine supersensitivity and elevated dopamine D2(high) receptors

Increasing the density of the D2L receptor and manipulating the receptor environment are required to evidence the partial agonist properties of aripiprazole

The clinical efficacy of aripiprazole in the treatment of psychosis relies on a partial agonism at D2 receptors. As the expression of this receptor differs physiologically between pre- and post-synaptic sites and is affected by pathological conditions or pharmacological treatments, it appears difficult to predict the clinical response to partial agonists. In addition, the response to this novel antipsychotic was shown to depend on the cell-line and the pathway analyzed, suggesting a functional selective profile at the D2 receptor. This study aims at examining the influence of receptor density and ionic environment on the pharmacological properties of aripiprazole. A cell line was developed in which the expression of the recombinant D2 receptor can be tightly manipulated using doxycycline and sodium butyrate. The potency and efficacy of aripiprazole and other reference D2 receptor ligands were examined in [35S]GTPγS binding assays, in buffers containing either NaCl or N-methyl-D-glucamine (NMDG) which is proposed to enhance G protein coupling. Increasing the density of D2 receptors considerably enhanced the [35S]GTPγS binding induced by dopamine and the full agonist NPA. In maximally induced cells, the agonist properties of the partial agonist (-)-3-PPP was revealed in a buffer containing NaCl, whereas the response to aripiprazole was not evidenced. Substituting NMDG for NaCl promoted the response to dopamine and (-)3-PPP and was proven efficient to reveal the partial agonist profile of aripiprazole. While NMDG substitution for NaCl strongly enhanced receptor-G protein coupling, these ionic manipulations are likely to influence receptor conformations, thereby modulating the activation of signaling pathways. Our data obtained with partial agonists acting at the D2 receptor suggest that these changes in the experimental conditions could contribute to reveal the functional selective profile of GPCR ligands. They also emphasize that the properties of functional selective ligands do not only depend on receptor density but also on the surrounding environment which likely differs between brain structures.

What are the optimal pharmacokinetic properties of antipsychotic medications?

Sensitization of the dopaminergic system has been proposed as a pathogenic mechanism of psychosis. It has been reported in sensitized animals that the proportion of high-affinity dopamine 2 (D2) receptors is increased, without changes in the total amount of D2 receptors. This increase induces an exaggerated postsynaptic transmission of the dopamine signal, which is attenuated by antipsychotic D2 antagonists. In this report, I simulated D2 receptor binding of dopamine and antipsychotics under pathologic state, and investigated pharmacological conditions that would return the increased dopamine binding back to normal levels in both resting and burst dopamine concentrations. I found that D2 antagonists with small Koff values at a concentration 2.5 times the Ki closely mimicked normal dopamine binding. Under these conditions, the apparent receptor occupancy of the drug was calculated as 70%.

Mutation of Semaphorin-6A disrupts limbic and cortical connectivity and models neurodevelopmental psychopathology

Psychiatric disorders such as schizophrenia and autism are characterised by cellular disorganisation and dysconnectivity across the brain and can be caused by mutations in genes that control neurodevelopmental processes. To examine how neurodevelopmental defects can affect brain function and behaviour, we have comprehensively investigated the consequences of mutation of one such gene, Semaphorin-6A, on cellular organisation, axonal projection patterns, behaviour and physiology in mice. These analyses reveal a spectrum of widespread but subtle anatomical defects in Sema6A mutants, notably in limbic and cortical cellular organisation, lamination and connectivity. These mutants display concomitant alterations in the electroencephalogram and hyper-exploratory behaviour, which are characteristic of models of psychosis and reversible by the antipsychotic clozapine. They also show altered social interaction and deficits in object recognition and working memory. Mice with mutations in Sema6A or the interacting genes may thus represent a highly informative model for how neurodevelopmental defects can lead to anatomical dysconnectivity, resulting, either directly or through reactive mechanisms, in dysfunction at the level of neuronal networks with associated behavioural phenotypes of relevance to psychiatric disorders. The biological data presented here also make these genes plausible candidates to explain human linkage findings for schizophrenia and autism.

Following the genes: a framework for animal modeling of psychiatric disorders

The number of individual cases of psychiatric disorders that can be ascribed to identified, rare, single mutations is increasing with great rapidity. Such mutations can be recapitulated in mice to generate animal models with direct etiological validity. Defining the underlying pathogenic mechanisms will require an experimental and theoretical framework to make the links from mutation to altered behavior in an animal or psychopathology in a human. Here, we discuss key elements of such a framework, including cell type-based phenotyping, developmental trajectories, linking circuit properties at micro and macro scales and definition of neurobiological phenotypes that are directly translatable to humans.

Shared vulnerability between seizures and psychosis in cocaine addiction?

Cocaine-induced seizures (CIS) and cocaine-induced psychosis (CIP) may be complications of acute cocaine intoxication. CIS could result from a kindling process, involving the glutamate NMDA receptor (NMDAR) phosphorylation state, which is enhanced by activation of the dopamine D1 receptor (D1R). CIP is considered to be more specifically associated with the activity of the dopamine D2 receptor (D2R). The authors describe the case of a 21-year-old woman who presented with recurrent CIP during a period of increased cocaine abuse that ended in two consecutive CIS. This case report may illustrate a possible overlap in the mechanisms underlying CIS and CIP, disclosing some subtle interactions occurring between dopaminergic and glutamatergic receptors during cocaine chronic intoxication. Chronic cocaine exposure usually induces the formation of a NMDAR-D2R complex, which seems to be linked to the usual clinical effects of the drug, but also causes complex formation not to occur in both D2R-based CIP and D1R-based CIS. To explain the case of this patient, we propose a pharmacological hypothesis based on a literature review and implying the lack of formation of this complex, which triggers CIP and CIS. On a more practical level, this case report also encourages practitioners to be aware of the possible co-occurrence of CIP and CIS in cocaine abusers, especially with respect to antipsychotic medications that could be administered in such situations.

Imaging of thalamocortical dysrhythmia in neuropsychiatry

Abnormal brain activity dynamics, in the sense of a thalamocortical dysrhythmia (TCD), has been proposed as the underlying mechanism for a subset of disorders that bridge the traditional delineations of neurology and neuropsychiatry. In order to test this proposal from a psychiatric perspective, a study using magnetoencephalography (MEG) was implemented in subjects with schizophrenic spectrum disorder (n = 14), obsessive–compulsive disorder (n = 10), or depressive disorder (n = 5) and in control individuals (n = 18). Detailed CNS electrophysiological analysis of these patients, using MEG, revealed the presence of abnormal theta range spectral power with typical TCD characteristics, in all cases. The use of independent component analysis and minimum-norm-based methods localized such TCD to ventromedial prefrontal and temporal cortices. The observed mode of oscillation was spectrally equivalent but spatially distinct from that of TCD observed in other related disorders, including Parkinson's disease, central tinnitus, neuropathic pain, and autism. The present results indicate that the functional basis for much of these pathologies may relate most fundamentally to the category of calcium channelopathies and serve as a model for the cellular substrate for low-frequency oscillations present in these psychiatric disorders, providing a basis for therapeutic strategies.

Alpha-phenyl-N-tert-butylnitrone prevents oxidative stress in a haloperidol-induced animal model of tardive dyskinesia: investigating the behavioural and biochemical changes

Haloperidol (HP) is a widely prescribed antipsychotic drug used for the treatment of mental disorders. However, while providing therapeutic benefits, this drug also causes serious extrapyramidal side effects, such as tardive dyskinesia (TD). Upon chronic administration, HP causes behavioural supersensitivity to dopamine D2 receptor agonists, as well as the development of vacuous chewing movements (VCMs), in an animal model of human TD. Currently, a prevailing hypothesis to account for these behavioural abnormalities implicates oxidative stress. This study was undertaken to examine whether the free radical trapping agent, α-phenyl-N-tert-butylnitrone (PBN), can prevent the development of behavioural supersensitivity to dopamine D2 receptor agonists and the development of VCMs. Additionally, the study examined whether increased synthesis of apoptosis inducing factor (AIF) can result from HP-induced oxidative stress. Male Sprague-Dawley rats were treated with HP in conjunction with PBN, or its vehicle, for 4weeks. After a 24-hour washout period, behavioural observations were recorded along with the estimation of lipid peroxidation and antioxidant enzyme activities. The free radical trapping agent, PBN, prevented the development of behavioural supersensitivity, reduced lipid peroxidation and prevented the reduction of antioxidant enzyme activities. AIF concentrations at the mRNA and protein levels remained unchanged; therefore increased AIF gene expression is unlikely to be involved in HP-induced oxidative stress. The findings of the present study suggest the involvement of striatal free radicals in the development of behavioural supersensitivity, and free radical trapping agents, such as PBN, as possible options for the treatment of extrapyramidal side effects in humans.

Schizophrenia, "Just the Facts" 6. Moving ahead with the schizophrenia concept: from the elephant to the mouse

The current construct of schizophrenia as a unitary disease is far from satisfactory, and is in need of reconceptualization. The first five papers in our "facts" series reviewed what is known about schizophrenia to date, and a limited number of key facts appear to stand out. Schizophrenia is characterized by persistent cognitive deficits, positive and negative symptoms typically beginning in youth, substantive heritability, and brain structural, functional and neurochemical alterations including dopaminergic dysregulation. Several pathophysiological models have been proposed with differing interpretations of the illness, like the fabled six blind Indian men groping different parts of an elephant coming up with different conclusions. However, accumulating knowledge is integrating the several extant models of schizophrenia etiopathogenesis into unifying constructs; we discuss an example, involving a neurodevelopmental imbalance in excitatory/inhibitory neural systems leading to impaired neural plasticity. This imbalance, which may be proximal to clinical manifestations, could result from a variety of genetic, epigenetic and environmental causes, as well as pathophysiological processes such as inflammation and oxidative stress. Such efforts to "connect the dots" (and visualizing the elephant) are still limited by the substantial clinical, pathological, and etiological heterogeneity of schizophrenia and its blurred boundaries with several other psychiatric disorders leading to a "fuzzy cluster" of overlapping syndromes, thereby reducing the content, discriminant and predictive validity of a unitary construct of this illness. The way ahead involves several key directions: a) choosing valid phenotype definitions increasingly derived from translational neuroscience; b) addressing clinical heterogeneity by a cross-diagnostic dimensional and a staging approach to psychopathology; c) addressing pathophysiological heterogeneity by elucidating independent families of "extended" intermediate phenotypes and pathophysiological processes (e.g. altered excitatory/inhibitory, salience or executive circuitries, oxidative stress systems) that traverse structural, functional, neurochemical and molecular domains; d) resolving etiologic heterogeneity by mapping genomic and environmental factors and their interactions to syndromal and specific pathophysiological signatures; e) separating causal factors from consequences and compensatory phenomena; and f) formulating or reformulating hypotheses that can be refuted/tested, perhaps in the mouse or other experimental models. These steps will likely lead to the current entity of schizophrenia being usefully deconstructed and reconfigured into phenotypically overlapping, but etiopathologically unique and empirically testable component entities (similar to mental retardation, epilepsy or cancer syndromes). The mouse may be the way to rescue the trapped elephant!

Dopamine receptor homooligomers and heterooligomers in schizophrenia

Over the past two decades the dopamine D2 receptor has been undoubtedly the most widely studied dopamine receptor for the therapeutic treatment of schizophrenia, as the majority of antipsychotics exhibit antagonism at this receptor. However, the cognitive symptoms of the disorder are mostly resistant to the majority of available antipsychotic treatments and, as a result, there is a critical need to develop novel therapies that ameliorate all symptoms. The recognition that dopamine receptors, such as all G protein-coupled receptors (GPCRs), exist as oligomeric complexes has provided new avenues for drug design in the search for novel therapies. Furthermore, that it is now known that dopamine receptors can form heteromers, such as the dopamine D1-D2 receptor heteromer, with pharmacology and function distinct from its constituent receptors, has significantly expanded the range of potential drug targets. The aim of this review is to discuss the therapeutic relevance of these dopamine receptor oligomers to schizophrenia and to address the potential value of dopamine receptor heteromers in the search for new therapeutic strategies.

Treatment of the psychostimulant-sensitized animal model of schizophrenia

Behavioral sensitization to psychostimulants in rodents is associated with the alteration of dopaminergic neurotransmission, and has been proposed as a useful model of schizophrenia due to its progressively intensifying, easily relapsing, and long-lasting features. Pharmacological treatments that reverse the established sensitization may have potential therapeutic values for schizophrenia. The present aim is to review pharmacological treatments that induce the reversal of established sensitization to psychostimulants. In addition, we discuss possible mechanisms for the reversal of sensitization. Reversal of sensitization is induced by chronic dopamine D1 receptor agonism, D2 or D1/D2 receptor agonism combined with mild N-methyl-D-aspartate (NMDA) receptor antagonism or serotonin (5-HT(2A) or 5-HT(3) ) receptor antagonism, 5-HT(1A) receptor agonism, and 5-HT(2A) or 5-HT(3) receptor antagonism. Chronic treatments with these drugs likely adjust altered dopaminergic neurotransmission in sensitized animals. Especially, chronic dopamine D1 receptor agonism, which may adjust mesolimbic hyperdopaminergic and mesocortical hypodopaminergic functions in sensitized animals, is an attractive therapeutic approach for schizophrenia.